2011
SOUTHEASTERN NATURALIST
10(2):251–266
Cave-obligate Biodiversity on the Campus of Sewanee:
The University of the South, Franklin County, Tennessee
Groves B. Dixon1 and Kirk S. Zigler1,*
Abstract - The southern Cumberland Plateau in Tennessee and Alabama has the greatest
diversity of cave-obligate animals in the United States. The University of the South in
Franklin County, TN is one of the largest private landholders on the southern Cumberland
Plateau. Its 13,000-acre campus has more than 30 caves and is underlain by more than
14 km of horizontal passageways. We examined the biodiversity of cave animals on the
campus at the species level and at the genetic level. Through a survey of seven caves on
the campus, we identi¿ed 24 cave-obligate species, including two new county records.
This total accounts for half of the cave-obligate species reported for Franklin County.
For our genetic analysis, we selected six diverse taxa (two millipedes, a beetle, a Ày,
an aquatic isopod, and a spider) that were collected from multiple caves, and compared
their mitochondrial cytochrome oxidase I gene sequences. Across the six taxa we found:
(1) low genetic diversity within caves (mean nucleotide diversity within caves across
all taxa: 0.25%), (2) high genetic divergence between caves (divergence between caves
within taxa ranged from 2.5%–10.9%, with two exceptions), and (3) little evidence for
gene Àow between caves (FST between caves within taxa > 0.57, with one exception).
Thus, the campus supports tremendous species diversity, and even more remarkable genetic diversity within those species on a small geographic scale (no studied caves were
>7 km apart). The divergence between cave populations and lack of gene Àow between
them that we observed across a range of taxa highlight the importance of cave conservation on a regional scale.
Introduction
Obligate cave-dwelling animals, both terrestrial (troglobites) and aquatic
(stygobites), represent a variety of taxa that have drawn the attention of scientists for centuries. They typically share a suite of convergent morphological
characteristics, including eyelessness, pigment loss, appendage elongation,
and increased extravisual senses, collectively termed “troglomorphy” (reviewed in Culver and Pipan 2009). Cave-obligate organisms generally have
small ranges (Culver et al. 2000), and many are known from only a single
cave (e.g., 45% of eastern United States cave species are single-cave endemics (Christman et al. 2005)). In addition to their small range sizes and high
rates of endemism, cave-obligate animals are restricted to karst (cave-bearing)
regions, with the result that cave-obligate biodiversity is concentrated within
relatively few geographical areas. Across nine major karst regions of the United States, the Appalachians and the Interior Low Plateaus have the most cave
1
Department of Biology, Sewanee: The University of the South, 735 University Avenue,
Sewanee, TN 37383. *Corresponding author - kzigler@sewanee.edu.
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biodiversity (Culver et al. 2003). The southern edges of these regions meet at
the southern Cumberland Plateau in Tennessee and Alabama.
Figure 1. Extent of the southern Cumberland Plateau in Tennessee and Alabama. County
borders and names are indicated, and the campus of The University of the South in Franklin County, TN is outlined in grey. Higher altitudes are designated by darker shades.
2011
G.B. Dixon and K.S. Zigler
253
The southern Cumberland Plateau supports the greatest richness of caveobligate species in North America (Culver et al. 2000), is a center of endemism
for cave species (Christman et al. 2005), and is a biodiversity hotspot for cave
fauna on a global scale (Culver et al. 2006). It is a region of high cave density
and high temperate productivity, both of which likely contribute to high levels of
cave biodiversity (Culver et al. 2006). The campus of The University of the South
(known as “Sewanee”) is located in the northeast corner of Franklin County, TN,
and covers 13,000 acres of this region (Fig. 1). As the privately held property of
an educational institution, the Sewanee campus offers a unique opportunity for
the study and conservation of cave biodiversity. Despite this, there has been no
systematic survey of Sewanee’s cave-obligate biodiversity.
Most studies of cave biodiversity have focused solely on species diversity.
On the southern Cumberland Plateau, a series of surveys (Lewis 2005; Peck
1989, 1995) and taxonomic revisions (Buhay and Crandall 2008, Lewis 2009,
Shear 2010) have provided a well-developed picture of the species diversity of
cave-obligate taxa for the region. Recently, several studies have examined the
genetic diversity of specific cave-obligate taxa in this region. These include
two studies on cave crayfish (Buhay and Crandall 2005, Buhay et al. 2007),
one on Tennessee cave salamanders (Niemiller et al. 2008), and one on a cave
spider (Snowman et al. 2010). These studies provide insights into the population genetics and evolutionary history of these cave-obligate taxa. Buhay
and Crandall (2005) and Buhay et al. (2007) emphasized the large population
sizes and extensive gene flow observed for the cave crayfish, Niemiller et al.
(2008) highlighted evidence for speciation of the Tennessee cave salamanders
in the presence of gene flow with surface salamander species, and Snowman
et al. (2010) found significant divergence and little evidence for gene flow between cave spider populations. Undoubtedly, studies of genetic diversity are
an important complement to studies of species diversity in caves (reviewed in
Porter 2007).
In this study, we examined the biodiversity of cave-obligate animals on the
Sewanee campus at the species level and at the genetic level. We ¿rst surveyed
seven caves on the campus. We then selected six diverse taxa (two millipedes, a
beetle, a Ày, an aquatic isopod, and a spider) that were collected from multiple
caves, and compared their mitochondrial cytochrome oxidase I gene sequences.
Although Sewanee has a large campus, all the caves we surveyed were within
7 km of one another, allowing us to examine gene Àow and population divergence on a small scale. Previous genetic analyses on a troglobitic spider from
Sewanee’s campus found that cave populations were isolated and genetically divergent (Snowman et al. 2010). By studying diverse taxa, we tested whether cave
populations are generally isolated from one another, and whether high levels of
intraspeci¿c diversity between caves are general characteristics of cave-obligate
species, or if they are a unique trait of a single cave-obligate lifestyle, morphology, or taxon.
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Materials and Methods
Sample sites
We surveyed seven caves on the campus of The University of the South. The
caves were selected on the basis of location, size, and accessibility. The sampled
caves represent the six largest horizontal caves found within the campus, with
a total of §6 km of horizontal passageways. In addition, we surveyed TD Cave,
a small cave located near Solomon’s Temple Cave. We also gathered previously
reported data on Dry Cave and Lost Cove Cave, which are located just outside the
northern and southern borders of the campus, respectively, and Wet Cave, which
runs beneath the campus, although the entrance is located outside of it. With the
exception of TD Cave, which is not listed by the Tennessee Cave Survey (TCS),
we refer to all caves by their TCS names. To protect sensitive cave habitats, cave
locations are not disclosed in this paper.
Field methods
Sampling was conducted between 30 January 2009 and 2 February 2010.
We visited each cave 4 to 6 times. Visits generally lasted 1 to 2 hours. Our
sampling methods included hand collecting, pitfall traps, and Berlese extraction from litter samples. Hand collected samples were stored temporarily in
reagent alcohol (90.25% ethanol, 4.75% methanol). For pitfall traps, we used
50- and 250-mL plastic cups. We filled the bottoms of these with reagent alcohol, and covered the openings with 1.3-cm wire mesh to prevent disruption by
larger fauna, or their accidental capture. Pitfall traps were usually baited with
Limburger cheese, smeared around the inner edge of the cup, though in several
cases, cow dung proved an effective substitute. Traps were dug into banks,
or wedged between rocks as to allow crawling access to the lip. They were
left for two to five days before recovery on a return visit. Litter samples were
transferred to the lab in plastic bags, and placed in Berlese funnels for invertebrate extraction. Cave-obligate salamanders and crayfish were observed and
recorded, but were not collected.
Sample identification
In the lab, specimens were sorted and stored in 95% ethanol at -20 °C. We
identi¿ed as many species as possible using available literature. If we could not
identify a species con¿dently, we deferred to recognized experts (see Acknowledgments). Scienti¿c nomenclature including authorities for all species collected
in this study are listed in Table 2.
DNA extraction, amplification, and sequencing
We selected six cave-obligate taxa for genetic analysis: Pseudotremia minos (Russell Cave Millipede), Pseudotremia barri (Barr’s Cave Millipede),
Ptomaphagus hatchi (Hatch’s Cave Fungus Beetle), Spelobia tenebrarum (Cave
Dung Fly), Caecidotea bicrenata (Two-toothed Cave Isopod), and Nesticus
barri (Barr’s Cave Spider). These were chosen for their abundance in our sample
sites, their responsiveness to our sequencing methods, and their taxonomic and
2011
G.B. Dixon and K.S. Zigler
255
morphological variety. For Barr’s Cave Spider, we supplemented sequences already collected from two caves on the campus by Snowman et al. (2010).
For DNA extractions, we used the DNeasy Blood and Tissue kit (Qiagen
#69506) according to the manufacturer’s protocol. DNA extractions were
examined by electrophoresis to ensure success before amplification was attempted. We amplified fragments of the mitochondrial cytochrome oxidase I
(COI) gene using the polymerase chain reaction (PCR). Our PCRs were 30
ȝL in volume: 15 ȝL AmpliTaq Gold PCR Master Mix (Applied Biosystems
#4316753), 12 ȝL distilled water, 1 ȝL each of the selected 5’ and 3’ primers at 10 ȝm concentration, and 1 ȝL of extracted DNA. All amplifications
were originally conducted using the universal COI primers HCOI-2198 and
LCOI-1490 (Folmer et al. 1994). After a successful sequence was acquired
for a species, we developed species-specific primers to increase the efficiency
of our amplification and sequencing reactions (Table 1). Our PCRs ran as
follows: 5 min at 95 °C; then 35 cycles of 15 sec at 95 °C, 15 sec at 45 °C,
and 1 min at 72 °C; with a final 7 min at 72 °C. When using species-specific
primers, we increased the annealing temperature to 50 °C. PCRs were examined by electrophoresis. Successful PCRs were purified (Qiagen #28106) and
sequenced on both strands. All sequences have been submitted to GenBank
(Accession #HM245857–HM245919).
Sequence analysis
COI fragments were aligned and edited using Sequencher 4.9 (Gene Codes
Corporation, Ann Arbor, MI). Variable sites were veri¿ed manually from chromatograms, and primers were excluded from all alignments. No indels were
present. To assess the level of genetic diversity of populations within individual
Table 1. Focal taxa, their common names, and the primer combinations which gave the most consistent results for each. All sequences were ¿rst ampli¿ed using the universal primers HC01-2198
and LC01-1490 (Folmer et al. 1994) before development of species-speci¿c primers. The Nesticus
barri primers are from Snowman et al. (2010).
Species
Primer
Sequence (5'-3')
Pseudotremia barri
(Barr's Cave Millipede)
HC01-tremia
LC01-1490
GTTGATATAAAATTGGGTCCCCTCC
GGTCAACAAATCATAAAGATATTG
Pseudotremia minos
(Russell Cave Millipede)
HC01-tremia
LC01-1490
GTTGATATAAAATTGGGTCCCCTCC
GGTCAACAAATCATAAAGATATTG
Ptomaphagus hatchi
(Hatch’s Cave Beetle)
HC01-ptom
LC01-ptom
GGGACATCCTTAAGACTTTTAATTC
GCTGGTAAAGAATTGGATCCCC
Spelobia tenebrarum
(Cave Dung Fly)
HC01-ten
LC01-ten
GAACTTTATATTTTATATTTGGGGC
GTCTCCTCCACCAGCAGGGTC
Caecidotea bicrenata
(Two-toothed Cave Isopod)
HC01-caec
LC01-caec
AGGGTCCCTCCCCCCTGGGG
GGGGCTTGAGCCGGAAGAGTCGG
Nesticus barri
(Barr’s Cave Spider)
HC01-2198
LC01-barri
TAAACTTCAGGGTGACCAAAAAATCA
GGACTTTGTATTTTATTCTTGGGTC
Order
Species
Anthrobia monmouthia Tellkampf
Liocranoides archeri Platnick
Nesticus barri Gertsch
Phanetta subterranea (Emerton)
Pseudoscorpionida
Hesperochernes mirabilis (Banks)
Kleptochthonius magnus Muchmore
Kleptochthonius tantalus Muchmore
Opiliones
Tolus appalachius Goodnight & Goodnight
Diplopoda
Callipodida
Tetracion jonesi Hoffman
Chordeumatida
Pseudotremia barri Lewis
Pseudotremia minos Shear
Scoterpes stewartpecki Shear
Scoterpes ventus Shear
Entognatha
Collembola
Pseudosinella pecki Christiansen & Bellinger
Pseudosinella hirsuta (Delamare)
Pseudosinella spinosa (Delamare)
Arachnida
Araneae
Class
S
L
LC
X
X
X
X
X
X
X
X
X
X
X
BB
X
X
X
X
GV
X
X
X
X
X
X
MC
S
X
X
X
X
TD
X
X
X
X
ST
S
L
L
L
WC
X
X
X
X
X
WS
L
S
L
X
L
L
L
L
DC
Southeastern Naturalist
X
?
X
X
X
BH
Cave
Table 2. Cave-obligate species found in and just outside Sewanee’s campus and the caves they inhabit. X designates observations made in this study, L designates records from Lewis (2005), and S designates records from Shear (2010). Uncon¿rmed records are indicated with a ‘?’. Caves are listed from south to
north and abbreviated as follows: LC = Lost Cove Cave, GV = Grapeville, BB = Buckets of Blood, BH = Sewanee Blowhole, MC = Miller Cave, TD = TD
Cave, ST = Solomon’s Temple, WC = Wet Cave, WS = Walker Springs, DC = Dry Cave.
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Diptera
Coleoptera
Diplura
Order
Total per cave:
Amphibia
Caudata
Turbellaria
Tricladida
Amphipoda
Decapoda
Malacostraca
Isopoda
Insecta
Class
Table 2, continued.
Gyrinophilus palleucus McCrady
Sphalloplana percoeca (Packard)
Stygobromus sp.
Orconectes australis australis (Rhoades)
Caecidotea bicrenata bicrenata (Steeves)
Spelobia tenebrarum (Aldrich)
Pseudanopthalmus humeralis Valentine
Pseudanopthalmus intermedius Valentine
Ptomaphagus hatchi Jeannel
Subterrochus ferus (Park)
Subterrochus steevesi Park
Litocampa cookei (Packard)
Litocampa valentinei (Conde)
Species
4
L
L
LC
12
X
X
X
X
GV
11
X
X
X
X
BB
11
X
?
X
X
X
X
BH
7
X
MC
6
X
TD
Cave
8
X
X
X
X
ST
10
L
L
L
L
L
X
WC
11
X
X
X
X
X
X
WS
17
L
L
L
L
L
L
L
L
L
DC
2011
G.B. Dixon and K.S. Zigler
257
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Vol. 10, No. 2
caves, we calculated haplotype diversity (h), and nucleotide diversity (ʌ) for
every cave population for which we had two or more sequences, using DnaSP
v.5 (Librado and Rozas 2009). Transition, tranversion, silent, and replacement
nucleotide substitutions for each species were counted using MEGA v.4 (Tamura
et al. 2007).
To examine variability between cave populations, we calculated mean pairwise divergence (DXY) and population pair-wise FST using DnaSP. FST comparisons
were only conducted between populations with three or more representative sequences. We also assessed variability and population structure for each species
as a whole. For these assessments, we constructed phylogenetic networks using
TCS 1.21 (Clement et al. 2000), and compared haplotype and nucleotide diversity calculated for the entirety of our samples for each species.
Results
Species diversity
We observed 24 cave-obligate species within the campus of The University of
the South. There were six arachnids, ¿ve diplopods, four insects, four non-insect
hexapods, three malacostracans, one turbellarian, and an amphibian (Table 2). A
total of 19 troglobites and 5 stygobites were observed. Only Ptomaphagus hatchi
(Hatch’s Cave Beetle) was found in every cave sampled. Five species were found
only in a single cave: Gyrinophilus palleucus (Tennessee Cave Salamander); a
cavernicolous pselaphid beetle, Subterrochus steevesi; the troglobitic Anthrobia
monmouthia (Mammoth Cave Spider); the cave Àatworm, Sphalloplana percoeca; and an unidenti¿ed amphipod, Stygobromus sp.
Previous surveys and taxonomic revisions identi¿ed 45 described caveobligate species (and several undescribed species) for the entirety of Franklin
County, TN (Culver et al. 2000; Lewis 2005, 2009; Shear 2010). Our survey increased this total to 47 with the observation of two previously unrecorded species
(Subterrochus steevesi and Anthrobia monmouthia). Subterrochus steevesi was
previously known only from Marshall County, AL (Chandler 1997, Park 1960).
Anthrobia monmouthia is known from scattered collections from Kentucky, Tennessee, Virginia, and Alabama (Miller 2005).
An average of ten cave-obligate species were found in each cave on the
campus (Table 2). Two species identifications from the Sewanee Blowhole
are unconfirmed. We collected only immature Scoterpes individuals from this
cave and so are unable to confirm the species. Other caves on the southern side
of campus have S. stewartpecki, but it is also possible that these individuals
represent the southernmost record of S. ventus (Table 2; Shear 2010). We also
observed, but did not collect, a troglobitic crayfish in this cave. This specimen
was probably Orconectes australis australis (Rhoades) (known from caves on
the north side of campus), but as it was found on the southern side of campus,
it may be Cambarus hamulatus (Cope) (known from caves in southern Franklin
County) (Buhay and Crandall 2005, Buhay et al. 2007). We also collected an
unidentified amphipod of the genus Stygobromus in Solomon’s Temple cave.
2011
G.B. Dixon and K.S. Zigler
259
Five species of Stygobromus are reported from Franklin County, three of which
are undescribed (Lewis 2005).
Intraspecific genetic variation
We acquired COI sequences from 69 individuals, and found 29 unique haplotypes among our six species of interest. Sequences ranged from 563 to 633 bp
of the COI coding region. No stop codons or indels were observed (Table 3).
Comparisons within species revealed a total of 168 variable sites: 156 were silent substitutions, and 12 were replacement substitutions. Across these sites we
observed 129 transition mutations and 41 transversion mutations, with 2 variable
sites showing both a transition and transversion (Table 3).
Nucleotide diversity (ʌ) of populations within caves across all species was
generally low (mean ʌall populations = 0.25%) but varied across caves and taxa
(Table 4). Seven populations were represented by a single haplotype, with nucleotide and haplotype diversities of zero. For populations represented by multiple
halplotypes, haplotype diversity (h) ranged from 0.39 to 0.84 (Table 4). Haplotype diversities across all caves were high (mean h = 0.681), with ¿ve of our six
focal species showing values >0.50.
Population structure
Divergence between populations averaged 3.7% and varied widely across
species and the caves compared (Table 5). Notably, the sampled populations of
C. bicrenata (from Walker Springs and Buckets of Blood), showed a divergence
of 10.9%, whereas populations of other species, such as those of S. tenebrarum
and P. minos, showed divergences of 0.3% and 0.4% between caves (Table 5).
Interspeci¿c divergence between P. minos and P. barri was 3.7%, which was
similar to most intraspeci¿c population comparisons.
Of the 29 haplotypes observed across our six focal taxa, 25 were restricted
to a single cave (Fig. 2). In three of the four cases in which haplotypes were
shared between caves, the caves are within 3.5 km of one another (between TD
Table 3. Focal taxa and their sequencing data. Number of individuals sequenced (n), the number
of base pairs sequenced (bp), the number of conserved and variable sites, the number of transition
(Transi.), transversion (Transv.), silent, and replacement (Repl.) mutations observed, and the number of stop codons present. There were two instances (marked with an asterisk) when a transition
and a transversion were found at a single variable site.
Species
Pseudotremia barri
Pseudotremia minos
Ptomaphagus hatchi
Spelobia tenebrarum
Caecidotea bicrenata
Nesticus barri
Total
n
bp
9
10
21
15
6
8
69
626
626
563
589
573
633
Conserved Variable
Stop
sites
sites Transi. Transv. Silent Repl. Codons
620
620
529
543
518
612
3442
6
6
34
46
55
21
168
4
5
30*
26*
45
19
129
2
1
5*
21*
10
2
41
6
6
34
39
53
18
156
0
0
0
7
2
3
12
0
0
0
0
0
0
0
Vol. 10, No. 2
Southeastern Naturalist
260
and Solomon’s Temple in Pseudotremia barri; between TD, Solomon’s Temple,
and Walker Springs in Ptomaphagus hatchi; and between Grapeville and Buckets
of Blood in Nesticus barri). In one case, the caves are on opposite sides of the
campus (between Walker Springs and Buckets of Blood in Spelobia tenebrarum)
(Fig. 2). FST values between caves were high (mean FST = 0.68), and only three
of eight comparisons had FST less than 0.90 (Table 5). The lowest of these was
between the Buckets of Blood and Walker Springs populations of S. tenebrarum
(FST = 0.10; Table 5).
Discussion
Species diversity and distributions
The campus of The University of the South supports an exceptionally rich
collection of troglobitic and stygobitic species. As noted by Culver et al.
(2000), the southern Cumberland Plateau is particularly rich in troglobitic
Table 4. Summary of genetic diversity data for focal taxa. For caves for which there were at least
two representative sequences, the number of individuals sequenced (n), number of haplotypes per
cave population, haplotype diversity and its standard deviation (h), and nucleotide diversity and its
standard deviation (ʌ) are reported. The “All” rows include all sequences for the species, including
those from caves with fewer than two representative sequences not listed in the table.
Species
Cave
n
# haplotypes
h
ʌ
Pseudotremia barri
Walker Springs
All
7
9
1
2
0
0.389 ± 0.164
0
0.0038 ± 0.0016
Pseudotremia minos
Grapeville
Buckets of Blood
All
3
7
10
1
4
5
0
0.810 ± 0.130
0.844 ± 0.080
0
0.0032 ± 0.0006
0.0033 ± 0.0004
Ptomaphagus hatchi
Grapeville
Buckets of Blood
Walker Springs
All
7
4
7
21
4
3
3
11
0.714 ± 0.181
0.833 ± 0.222
0.667 ± 0.160
0.890 ± 0.049
0.0047 ± 0.0016
0.0021 ± 0.0007
0.0186 ± 0.0064
0.0239 ± 0.0017
Spelobia tenebrarum
Grapeville
Buckets of Blood
Walker Springs
Sewanee Blowhole
All
2
5
6
2
15
1
3
2
1
6
0
0.800 ± 0.164
0.533 ± 0.030
0
0.857 ± 0.003
0
0.0032 ± 0.0008
0.0019 ± 0.0006
0
0.0231 ± 0.0067
Caecidotea bicrenata
Buckets of Blood
Walker Springs
All
4
2
6
1
1
2
0
0
0.533 ± 0.019
0
0
0.0583 ± 0.0188
Nesticus barri
Grapeville
Sewanee Blowhole
All
3
4
8
2
1
3
0.667 ± 0.314
0
0.571 ± 0.094
0.0011 ± 0.0005
0
0.0182 ± 0.003
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G.B. Dixon and K.S. Zigler
261
species and is less rich in stygobitic species. This description correlates with
our findings of 19 troglobitic species and 5 stygobitic species. Culver et al.
(2000) noted that only 8 out of 3112 counties in the United States had more
than 20 troglobitic species, yet here, on only 13,000 acres, we identified 19.
The 24 total species found represent half of the 47 total cave-obligate species
known from Franklin County. Also notable is the exceptionally species-rich
Dry Cave, located just to the north of the campus. Seventeen cave-obligate
species have been found in Dry Cave (Table 2), which is an impressive
number given that only 36 caves worldwide are known to have 20 or more
cave-obligate species (Culver and Pipan 2009).
The Sewanee campus is transected by species range borders for eight of
the 24 cave-obligate species we observed (Table 2). This large proportion
reflects the small range sizes characteristic of troglobionts (Christman et al.
2005, Culver et al. 2000). The campus covers the southern range borders for
Scoterpes ventus, Pseudotremia barri, Pseudanopthalmus humeralis, and
Pseudanopthalmus intermedius, and the northern range borders for Scoterpes
stewartpecki, Pseudotremia minos, Nesticus barri, and Tetracion jonesi
(Table 2). The diplopod genera Scoterpes and Pseudotremia represent the
clearest range borders, with one species from each genus found in caves on
the northern side of the campus, and a second species from each genus found
in caves on the southern side of the campus. For these genera, the distributions of the northern and southern species pairs on the campus are identical
(Table 2). Whether this correlation is coincidental or the result of a shared
evolutionary history remains to be determined.
Table 5. Genetic divergence between populations for the focal taxa. Pair-wise distances (below
diagonal) between cave populations represented by at least two sequences, and pair-wise FST (above
diagonal, in bold) between caves represented by at least three sequences for Pseudotremia minos
and P. barri (Ps), Ptomaphagus hatchi (Pt), Spelobia tenebrarum (Sp), Caecidotea bicrenata (Ca),
and Nesticus barri (Nb).
Cave
Grapeville
Grapeville
Buckets of Blood
0.577 (Ps)
0.901 (Pt)
Buckets of Blood
0.004 (Ps)
0.037 (Pt)
0.044 (Sp)
Sewanee Blowhole
0.071 (Sp)
0.032 (Ne)
0.046 (Sp)
Walker Springs
0.036 (Ps)
0.030 (Pt)
0.045 (Sp)
0.038 (Ps)
0.027 (Pt)
0.003 (Sp)
0.109 (Ca)
Sewanee Blowhole
Walker Springs
1.000 (Ne)
1.000 (Ps)
0.901 (Pt)
0.957 (Ps)
0.597 (Pt)
0.104 (Sp)
0.046 (Sp)
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Gene flow between caves
Troglobitic and stygobitic populations from a variety of taxa show high
levels of genetic isolation between caves, even when they are found within
a relatively small area (no caves sampled in this study are more than 7 km
apart). This isolation is evident from the high proportion of haplotypes that
are not shared between caves (25 of 29), the high pair-wise FST values (>0.55,
with one exception) between caves (Table 5), and the restrictive clustering of
haplotypes by cave (Table 4, Fig. 2).
Even in cases where haplotypes are shared between caves, there is evidence that gene flow is limited by distance. In three of the four cases where
haplotypes are shared between caves, the caves are adjacent to one another.
The sole exception is in Spelobia tenebrarum, where a haplotype is shared
between caves on opposite sides of the campus. As the only cave-obligate fly
in North America, S. tenebrarum is clearly exceptional. It has a large range
(Marshall and Peck 1985) and is one of only nine cave-obligate species recorded from more than 30 counties in the eastern United States (Christman
Figure 2. Haplotype networks for six focal species: (A) Pseudotremia barri, (B) Pseudotremia minos, (C) Ptomaphagus hatchi, (D) Spelobia tenebrarum, (E) Caecidotea
bicrenata, and (F) Nesticus barri. Circles represent haplotypes, and lines connecting
them represent single nucleotide differences. Black bars and junctions between lines
indicate unsampled and/or extinct haplotypes. Circle area is proportional to the number
of sampled individuals with that haplotype (ranging from 1 to 7). Labels within circles
indicate the cave (or caves) where that haplotype was found (abbreviated as in Table 1).
Haplotypes found in more than one cave are indicated by the presence of more than one
label within a circle. Unconnected networks or haplotypes differ by ten or more nucleotide differences from all others.
2011
G.B. Dixon and K.S. Zigler
263
and Culver 2001). Exactly how S. tenebrarum moves around is unclear, but it
must have more ability to do so than most cave-obligate species. Despite this,
our sample of S. tenebrarum shows evidence of isolation and divergence, as
two of the four caves we sampled have unshared haplotypes that do not connect to the network that contains the haplotype shared between Buckets of
Blood and Walker Springs (Fig. 2).
Divergence between caves
Despite low genetic diversity within caves (mean ʌall populations = 0.25%; Table 4)
there are high levels of intraspeci¿c divergence between caves (DXY • 2.7% for
12 out of 14 comparisons, Table 5). Craft et al. (2010), using COI sequences to
study lepidoptera from New Guinea, deemed intraspeci¿c divergence greater
than 2% as indicative of “cryptic lineage diversity”. According to this standard,
12 of our 14 intraspeci¿c population comparisons identi¿ed cryptically distinct
lineages. High levels of genetic diversity within species are also evident in the
haplotype networks. For only two of the six focal taxa did all of the observed
haplotypes connect into a single network (Fig. 2).
It is generally assumed that aquatic subterranean habitats are better connected than terrestrial ones, facilitating greater dispersal ability for stygobites
over troglobites (Culver et al. 2007, Lamoreaux 2004), and studies have shown
that some stygobites are capable of wide-ranging dispersal (Buhay and Crandall 2005, Danielopol et al. 1994). Our limited molecular results for the aquatic
isopod, C. bicrenata, were contrary to this pattern as they showed the greatest
divergence of any in our sample group (DXY = 10.9%; Table 5). This divergence
suggests that the two caves we sampled are not hydrologically connected, which
may be the case as the stream from one of the caves drains to the south and the
Tennessee River, whereas the other drains to the north and the Elk River. Further
examination of this species would be interesting, particularly in light of its large
range, which spans from northern Alabama through Tennessee, Kentucky, and
into southern Illinois (Lewis 1982).
Two other recent studies have examined COI diversity and divergence of
cave animals on a small geographic scale. Carlini et al. (2009) studied the
amphipod Gammarus minus Say, surveying COI diversity and divergence for
five cave populations, including four that are no more than 20 km apart. They
found low nucleotide diversity within populations (mean ʌ = 0.70%), high
divergence between cave populations (§5% for most comparisons between
caves), and isolation between caves. Snowman et al. (2010) studied Nesticus
barri across its four-county range on the southern Cumberland Plateau, and
found low COI diversity within caves, high divergence between caves (2–5%),
and isolation between cave populations. Thus, the findings of these studies are
consistent with our results, although we found these patterns on a smaller geographic scale, and for a variety of taxa. These results from small stygobitic
and troglobitic invertebrates contrast with those from larger stygobites studied in this area (crayfish and cave salamanders), which found mitochondrial
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haplotypes shared across counties (Buhay and Crandall 2005, Buhay et al.
2007, Niemiller et al. 2008). These large stygobites may be more mobile than
troglobites or smaller stygobites, which may explain the patterns observed.
Conclusion
We identified 19 troglobites and 5 stygobites from the campus of The
University of the South. For at least eight of these species, the campus is
a northern or southern range boundary. Across six diverse taxa we found:
(1) low genetic diversity within caves, (2) high genetic divergence between
caves, and (3) little evidence for gene flow between caves. These patterns were generally consistent across the taxa studied, regardless of their
mechanism of locomotion, ecological role, and habitat, suggesting these
characteristics are common for cave-obligate species from this region. Particularly notable is that we observed these patterns within a small geographic
area, where no surveyed cave was more than 7 km from another. Thus, the
campus supports tremendous species diversity, and even more remarkable
genetic diversity within those species on a very small scale. Our observation
of high genetic divergence between caves, and little gene flow between them,
highlights the importance of cave conservation on a regional scale, in order to
conserve as much genetic diversity of cave-obligate species as possible.
Acknowledgments
We thank T. Barr, C. Carlton, L. Ferguson, J. Lewis, P. Paquin, and W. Shear for
taxonomic assistance. We thank M. Williams, A. Sidik, G. Cooper, and J. Christopher
for assistance in the ¿eld, and J. Benson and D. Durig for caving advice. We also thank
N. Hollingshead, V. Moye, and A. Sidik for assistance with maps and ¿gures. N. Berner,
D. McGrath, A. Sidik, and A. Summers commented on the manuscript. This project was
supported by Faculty Research Support funds from Sewanee: The University of the South
and by a Yeatman Fellowship (to G.B. Dixon).
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